In commercial inkjet printing systems, a print media is physically transported through the printing system at a high rate of speed. For example, the print media can travel 650-1000 feet per minute. A commercial inkjet printing system can include multiple lineheads, with each linehead having one or more printheads. The printheads typically include multiple nozzle plates, with each nozzle plate having precisely spaced and sized nozzles. The cross-track pitch, measured as drops per inch or dpi, is determined by the nozzle spacing. The dpi can be as high as 600, 900, or 1200 dpi.
The print media can receive a large amount of ink during printing, especially with water-based ink or in high ink laydown regions of the printed content (e.g. a picture with a lot of dense black background). In turn, the aqueous component of the ink is absorbed into the print media and can cause the print media to swell and stretch, especially if the print media is under tension. Stretch is usually significantly higher in the direction of movement (i.e., the in-track direction) than in the cross-track direction.
Additionally, heat is typically applied at one or more locations in a printing system to dry the ink that has been applied to the print media. Drying of the print media can cause the print media to shrink. When the print media is heated in between lineheads, regions of the print media can be stretched and shrunk one or more times as the print media moves through a printing system.
Printing with several color planes in which each color record is printed sequentially requires color laydown correlation. Unanticipated or unaccounted for stretch or shrink in the print media can cause a loss of color correlation and can lead to blurry content or hue degradation. Additionally, printing on both sides of the print media usually requires front-to-back registration, and the second side of the print media is usually printed significantly later than the first side.
Visible patterns such as dots, lines and polygons are typically printed on the print media so that a high speed and high magnification camera can record the pattern to determine if there are deviations from a reference value. If there are deviations, in-track and cross-track compensation values can be calculated and used to adjust the position or speed of the print media or of the drops of ink. Such cameras are often costly and dedicated for imaging the visible patterns. The cameras are usually kept stationary to monitor for the dots, lines and polygon patterns, and can monitor only a limited portion of the print media. If more visible patterns are printed than there are cameras, the extra patterns are either ignored or the cameras are moved to different positions to analyze all of the patterns in a sequential manner. Moreover, the visible patterns of dots, lines or polygons are usually large (millimeter to centimeter in size) and printed on the edges of the print media or the edges of the printed content. The printed visible patterns must be trimmed away before the printed content is assembled into a final product, such as a magazine or book. Due to cost considerations, the number of digital cameras mounted in a printing system is usually kept to a minimum to control the overall cost of the printing system.